Spectrometers are well known for use in the measurement of both wavelength and bandwidth of light sources, e.g., lasers. The output of such a bandwidth meter for example is actually the result of measurement, e.g., utilizing a position sensitive detector, e.g., a photo diode array (“PDA”) as is well known in the art. The bandwidth meter has its own response function that modifies the spectrum being measured during the measurement of the bandwidth.
However, as the requirements for more narrow critical dimension line features on, e.g. Ultra-Large-Scale-Integration (“ULSI”) integrated circuit fabrication are ever increasing (decreasing critical dimensions), the demand for pure laser light at a particular short wavelength (Deep Ultraviolet—“DUV” and Extreme Ultraviolet—“EUV”) and with purity defined by very narrowly controlled bandwidth around the center wavelength, the etalons used for such measurements are becoming much more susceptible to error due to the laser bandwidths required approaching the same bandpass as practical etalons in use. For, example on a recently released product of Cymer, Inc., the owner by assignment of the present application, the XLA 100, an on-board bandwidth meter utilized an etalon with a bandpass of about 0.12 pm and the laser provided an output generally between about 0.1 pm and 0.18 pm, discounting bandwidth resonance. The convolution then distorts the measured laser light, e.g., in bandwidth, in ways that now make or will soon make measurements of, e.g., the full width half maximum (“FWHM”) insufficiently accurate for properly monitoring laser output.
Currently the output of the bandwidth meter used as an indication of the bandwidth of the measured laser light bandwidth is, e.g., a measure of a fringe width, e.g., at full width half maximum (“FWHM”) of a fringe produced by the etalon optics less some constant error value sometimes called Etalon Resolution (“ER”) or Etalon Correction (“EC”). This has been done, e.g., in lasers sold by the assignee of the present invention under the names of NL-7000 and ELS, employing EIS wave meters, utilizing a slope and intercept formulation of Spectrum FWHM A(fringe FWHM)+C, with A and C calculated in a calibration process, e.g., at the time of manufacture.
There is a need, therefore for an improved methodology.